Forum: F-35 Lightning II

As I'm sure some of you know, increasing an aircrafts angle of attack usually decreases the effectiveness of its vertical tail(s). At particularly high angles of attack, much of the vertical tail may be blocked from clean airflow by the fuselage. Also, the orientation of the rudder to the airflow at high alpha probably reduces its effectiveness as well.

I believe that this is where ventral fins come in, right? Since they are below the fuselage, they remain in clean air at high alpha and help the aircraft maintain lateral stability. A lot of fighters have had ventral fins. For example, the F-16, F-14, Su-27, MiG-23, MiG-21, J-10, Lavi, MiG 1.44, MiG-25, and XF8U-3 (with particularly large ones, although I think these were more for stability at high speeds rather than high alpha).

However, the F-35 does not have ventral fins (probably mostly for stealth reasons). Despite this, it has a much higher angle of attack limit (55 degrees) than the F-16 (25 degrees). How is it able to accomplish this? I imagine that the flight control system is probably quite advanced and has a lot to do with this. Then again, the vertical tails are also canted outward, which probably helps with high alpha, right (like the F/A-18, which also lacks ventral fins)?

Also, I think the F-16's alpha limit has more to do with sudden "pitch up" avoidance than a lack of lateral stability. Still, 55 degrees seems very impressive for the F-35, which doesn't have thrust-vectoring.

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Might I ask where you found that the F-35 can go up to 55 degrees? I don't doubt it, but it would be an interesting read if it came with an article. I think the F/A-18's high AOA comes from it's large Leading Edge Extensions (cobra hood). It's quite a bit larger than the F-16's and the F-35/F-22 have just about the same thing but it's built in to be stealthy. Perhaps that helps?

The F-16 has one tail and the F-35 has two. Throw in a perfectly shaped wing, leading edge flaps, and state of the art control laws then your stall AOA really jumps up.

Stall AoA has nothing to do with AoA limit. AoA limit is set by a number of factors - loss of pitch control (horizontal tail becomes ineffective), loss of directional control (can't keep the pointy end in front), for exmple. LMAggie is right that two tails contribute to higher AoA limt since the tails are not blanked out by the fuselage as much.

The ventrals on the F-16 do help some with directional stability at high AoA, but their primary contribution is at high mach, low AoA conditions. At those conditions, the vertical tail and rudder lose effectiveness due to aeroelastic effects, so the ventrals help maintain directional control.

It's interesting that three derivitives of the F-16 do not have ventrals - F-16XL, IDF (Taiwan), and T-50 (Korea). The XL wing helps with directional stability, and the other two don't have the high mach capability of the F-16, so have less need for ventrals.

Also, in high AOA the vertical stab is not necessarily ineffective. In many cases it becomes the most effective control surface about the longitudinal axis, more so than the ailerons.

In the F-16 the AOA limit is largely due to its relaxed static stability (CG and CL nearly aligned). The risk is that if the F-16 were to enter a stall with its CG being so far aft, it may become unrecoverable, or at the very least use up so much altitude to recover that it's not worth the risk. It wouldn't be surprising if the CG of the F-35 was far enough forward of the CL that it allowed for greater AOA without the increased risk. Also, another possibility is that the larger horizontal stabs, combined with the slanted vertical stabs, can create enough of a pitch moment at high AOA to compensate. Combine both of those with a generational advance in FLCS, and it seems that 55 deg may be quite possible.

just my Two Cents

dragorv wrote:

Might I ask where you found that the F-35 can go up to 55 degrees?

I'm pretty sure that I read it on this discussion board in a previous thread or perhaps in an article linked from a thead. Unfortunately, I don't know which thread.

Kryptid wrote:

dragorv wrote:

Might I ask where you found that the F-35 can go up to 55 degrees?

I'm pretty sure that I read it on this discussion board in a previous thread or perhaps in an article linked from a thead. Unfortunately, I don't know which thread.

I'm pretty sure it's a requirement, F-18 is pretty much unlimited AOA. Navy wants that!

This thread is useful: http://www.f-16.net/f-16_forum_viewtopic-t-12556.html
&
http://www.livescience.com/technology/0 ... -jets.html

spazsinbad wrote:

This thread is useful: http://www.f-16.net/f-16_forum_viewtopic-t-12556.html
&
http://www.livescience.com/technology/0 ... -jets.html

Might add that AOA stability is also provided by the chinned forebody and engine cowls. As Beesley comments; "It’s a fully maneuverable 50-degree airplane”.

High AOA probably useful in this condition:
http://www.youtube.com/watch?v=i1PLtCC_M-E
http://www.youtube.com/watch?v=g-8DFu-6 ... eature=fvw

B. Bolsøy
Oslo

Kryptid, found the article mentioning the 55 degree AoA: http://armoredd.com/home/archives/1561

F-35 Air Combat Skills Analyzed 6th March 2009 Article first appeared in Aviation Week & Space Technology

"The F-35’s ability to win an air-to-air engagement is drawing increased attention as the U.S. military and industry’s focus includes expanding the Joint Strike Fighter’s customer base beyond the core purchasing nations.

For years, prime contractor Lockheed Martin seemed content to promote the F-35’s “strike fighter” capabilities, if only to avoid competing against its other major fighter program, the F-22 Raptor. But with the F-22 not exportable, Lockheed Martin seems keen to talk up the F-35’s air combat skills to bolster its chances for new foreign military sales — namely, to Japan, Turkey and Greece.

The contractor tells Aviation Week that the JSF’s combination of stealth, multisensor situational awareness, advanced pilot-machine interface and basic aeromechanical performance make it a credible fighter aircraft, too. That is key to several other customers, who cannot afford the so-called high-low fighter mix on which the U.S., U.K. and Italian air forces are planning.

Lockheed Martin is focusing largely on the beyond-visual-range fight, with ranges greater than 18 naut. mi. that executives say will represent 62% of all aerial combat. Another 31% of engagements would fall into the 8-18-naut.-mi. transition range, and just 7% of fighting would be close-in combat where the airframe is stressed the most.

Lockheed Martin says it ran the F-35 through the Pentagon’s TAC Brawler simulation for air combat systems analysis, using what would be the “ideal” air combat configuration, taking the conventional-takeoff-and-landing F-35A, the only model designed to perform full 9g maneuvers.

The aircraft can also reach a 55-deg. angle of attack in trimmed flight, while most fighters, excluding the F/A-18, are limited to 30 deg. The exact performance of the current F-35A configuration — also known as the 240-4 — are classified. But a similar earlier standard (240-3) was credited with a maximum speed of Mach 1.67; acceleration from Mach 0.8 to Mach 1.2 at 30,000 ft. in 61 sec.; a top turning speed of 370 kt. at 9g and 15,000 ft.; and a sustained turn capability of 4.95g at Mach 0.8 and 15,000 ft. Moreover, an aircraft with those performance figures would carry two beyond-visual-range AIM-120 Advanced Medium-Range Air-to-Air Missiles (Amraams) in the internal weapons bay.

Yet, such performance numbers appear to leave the F-35 short of the kind of air-to-air capabilities provided by other combat aircraft, such as the Russian Su-30MKI or the European Typhoon. And even Lockheed Martin test pilots concede that the F-35 — although offering very high initial acceleration due to its powerful 42,000-lb.-thrust F135 engine — could start losing advantage at higher speed and altitude. This might be partly due to the aircraft’s large frontal area, which is designed to allow internal weapons carriage — meaning in a traditional quick-reaction intercept role, the F-35 may not be able to match rivals.

Nevertheless, Brawler modeling showed the F-35 could achieve a loss-exchange ratio better than 400% against its nearest “competitor,” according to Lockheed Martin executives. They demur about naming the competitor, but their comparison charts indicate it is the Sukhoi Su-30 or Typhoon.

That engagement ratio comes from the combination of F-35 characteristics, executives argue, including stealth, the performance of the APG-81 active electronically scanned array radar, sensor fusion using data links and the 360-deg. situational awareness afforded by the distributed aperture system of infrared and electro-optical sensors and electronic support measures.

In the meantime, and without discussing specific performance characteristics, Italian air force fighter pilots involved with the F-35 program tell Aviation Week that the aircraft’s performance falls “between the F-16 and the F/A-18 in terms of flight envelope — and is actually closer to the F/A-18, considering its high angle of attack and slow-speed maneuvering capabilities.”

The F-35A, with an air-to-air mission takeoff weight of 49,540 lb., has a thrust-to-weight ratio of 0.85 and a wing loading of 110 lb. per sq. ft. — not ideal for a dog-fighter. The F135 engine delivers 42,000 lb. thrust, and industry officials suggest that an F-35 entering an air-to-air engagement with 40% — or more than 7,275 lb. — of internal fuel will have a thrust-to-weight ratio of 1.09 and a wing loading of 83 lb. per sq. ft. Those figures describe an agile, albeit not top-end, fighter.

Still, one key feature, Lockheed Martin executives stress, is the very low observability designed into the JSF. Whereas the F-35 would carry its weapons internally, the Typhoon, Su-30, Saab Gripen or Dassault Rafale carry their missiles externally, thus increasing their radar signatures and degrading their on-paper air-to-air performance. The F-35 also accommodates more internal fuel, 8.3 tons, giving it greater endurance potential without external fuel tanks that would affect radar signatures.

Nevertheless, the F-35 may have notable weaknesses for pure air-to-air combat. For one, it is not designed to conduct engagements in a high-speed, high-altitude, sustained turning environment. Its high-speed cornering capability should help it to dodge an adversary’s beyond-visual-range missiles, though, particularly if German and U.K. air-to-air simulations on the kill probability of modern medium-range air-to-air missiles are accurate.

Those figures are part of the rationale for countries pursuing the ramjet-powered MBDA Meteor missile to supplant Amraams. Yet even in the Amraam world, Typhoons may have an edge over the F-35, since they could launch the missile at higher speed. Sukhoi Su-30s and the future T-50 are also being designed to maximize air-to-air missile performance that way.

Finally, while Lockheed Martin touts F-35 stealth as an advantage, it has its drawbacks, as well. The aircraft’s payload is limited as long as it wants to preserve its low-observable signature through internal carriage. That means having only four AIM-120s at its disposal. A study now underway could boost that total to six Amraams. Other weapons, including infrared-guided air-to-air missiles, would be carried externally, with plans for a “stealthy” JSF adaptation using a low-signature pylon design. Still, the radar signature would increase, as would drag, further reducing the F-35’s potential.

It is not clear how critical such perceived shortcomings truly are. Some pilots argue that in a dogfight, the air-to-air missile has more to do with the engagement’s outcome than does the aircraft."

Kryptid wrote:

As I'm sure some of you know, increasing an aircrafts angle of attack usually decreases the effectiveness of its vertical tail(s). At particularly high angles of attack, much of the vertical tail may be blocked from clean airflow by the fuselage. Also, the orientation of the rudder to the airflow at high alpha probably reduces its effectiveness as well.

Deuce has all the answers … He cracks me up Laughing

I can’t wait until they start departing F-35’s at Edwards, unless they are like T-38’s and F-111’s (one departure per jet Shocked

)

http://www.youtube.com/watch?v=VImEvFg3 ... re=channel

Forum: F-35 Lightning II

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